The Makara I think is not based on one single animal but seems more like a composite of different ones. Usually I've seen different versions of this creature and many have composite features of animals like dolphin, shark, crocodile, elephant, and deer. Its fluidity is apparent when I saw the Chinese version of it, which is heavily dragonified.

As far as the reason of this creature, maybe it has something to do with each Hindu deity having their own magical animal mount (the bird god Garuda for Vishnu, the elephant Airavata for Indra, etc). Since the Makara is the only major mount for the water god Varuna and the river goddess Ganga, maybe it's an inventive tie to the typical large beasts of the land associated with the water.

the word makara is the original term given to the mugger crocodile, and thus the makara of indian art was represented as a vahana of kamadeva and also i think ganga, as a crocodilian beast. later, it was given both fishlike and turtle-like features alternatively, sometimes as a chimera of all three. when the makara motif reached sri lanka, it was interpreted as a complete chimera, mainly with an elephant's trunk, fish scales, the ears of a pig, eyes of a monkey, teeth and jaws of a crocodile, wings of a bird ( in more modern depictions) and i think the paws of a lion. but these features are certainly not consistent and different artists interpret them differently.

not surprising, considering that the makara is after all, the vahana of the goddess Ganga...plus i realized something....what if t'sul k'alu and the si-te-ca were giant ground sloths? because i really can't think of any other strange, giant furry semi-bipedal North American mammals and anyway these lived alongside people. Plus the ground sloths would be mixed feeders and the trees are a relatively untouched food source in the continent currently and would occupy a rather low energy niche in the environment. Maybe the six fingers thing could be the result of tall tales, while the slanted eyes could be slanted stripes running across the animal's face. And with armour under their fur...well...? I mean, some of these legendary North American beasts are relatively easy to pin down.